HTH Dry Chlorine


In an outdoor pool the chlorine added can be adversely affected by sunlight dissipating the chlorine from the water even if no swimmers use the pool. To prevent this unnecessary loss, the chlorine can be stabilised with cyanuric acid. This may be achieved in two ways: 1. Use a non – stabilised chlorine donor such as sodium or calcium hypochlorite. (These chemicals contain no cyanuric acid). In this case cyanuric acid can be added separately and as required to maintain a residual of 50 – 70 mg/l. or 2. Use a product which already contains cyanuric acid, one of the chlorinated isocyanurates such as trichlor or dichlor as explained above. These chemicals add cyanuric acid to the water each time chlorine is added, CAUTION!!! This can lead to further problems without due care. As the chlorine kills bacteria introduced by bathers, more chlorine needs to be added. In this instance, each time chlorine is introduced then so too is more cyanuric acid or stabiliser. The chlorine is used up but the cyanuric acid stays in the pool and increases its ability to stabilise the chlorine against UV light. Once the cyanuric acid level reaches about the 200 mg/l level it will stabilise the chlorine to a point where it is rendered useless as a sanitiser. This is known as chlorine lock. It may appear from water tests carried out by the operator, that a perfectly satisfactory chlorine residual exists, but there may also be signs of algae on pool surfaces, green water or clarity is poor. This can be caused by excessive cyanuric acid, the operational maximum is 100 mg/l and the absolute maximum is 200 mg/l. The only way to remove cyanuric acid is to dilute the pool with fresh water. It is for these reasons that isocyanurates may not be the best product to use in an indoor pool. The reasoning is simple. Sunlight will not be a major factor in an indoor pool so the chlorine will not need to be stabilised. Further information on hth can be found in the hth Pool Manager's Handbook obtainable from us by request via the Contact us page

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